In the field of the machine tool, the requirements of the accuracy of finishing have been increased year by year, and recently an accuracy of submicron order is often required. In order to improve the accuracy of finishing, machine tools are developed so as to measure the dislocation of the tool relative to the spindle and the thermal expansion in the spindle.
As a first prior art, Japanese Unexamined Patent Publication (Kokai) No. 11-138392 discloses an NC machine tool which can measure, during the rotation of a tool, the cutting edge of the tool, the tool diameter, the tool length and the tool shape. The NC machine tool is provided with an optical tool measuring instrument which generates a fine linear beam between a projector and a receiver. A tool is mounted to the spindle, moved relative to the table in X-, Y- and X-axial directions to approach the beam in a predetermined direction. The center of the tool, the tool diameter and the tool length, defined in the NC program for the NC machine tool, are corrected on the basis of the X-, Y- and Z coordinates when the tool intercepts the beam.
As a second prior art, Japanese Unexamined Patent Publication (Kokai) No. 10-309653 discloses a machine tool in which the position of the cutting edge of a tool rotating in a high speed is accurately measured. In this machine tool, a reference block is fixed onto a top of a table. A displacement gauge, for measuring the distance to the reference block, is provided to a spindle housing. Another displacement gauge, for measuring the distance of the spindle end or a tool holder relative to the housing, is provided to the spindle housing. The displacement of the end of the spindle housing and the displacement of the spindle relative to the end of the spindle housing are obtained to correct the change in the position of the cutting edge of a tool.
In the first prior art, the position of the center of a tool, the tool diameter and the tool length is measured by the beam in non-contacting manner during the rotation of the tool to correct the machining program. However, the optical measurement has a problem that because the configuration of the tool tip is complex, the measured point is not clear in a the rotating tool and the accuracy of measurement is low. Another type of non-contacting distance measurement using such as a electrostatic capacity or eddy current also cannot solve the problem of the low measurement accuracy because the measure point is not clear in a rotating tool
In the second prior art, there is a problem that only the change in the tool length in the direction of the axis of the spindle or Z-axis can be measured but the apparent increase in the tool diameter due to a tilt or an offset of a tool relative to the spindle and the decrease in the tool diameter due to wear of the cutting edge of a tool cannot be measured.